You cannot disable in-app purchases on an Android device, but you can lock/password protect to restrict purchases. Open the Google Play app, then go to Menu, Settings then User Controls.
For an iOS/Apple device, you can disable in-app purchases by opening the Settings App, Tap Screen Time, Tap Content & Privacy Restrictions, then Switch the Content & Privacy Restrictions slide to on/green if you haven't already. Then Select iTunes & App Store Purchases and Tap Don't Allow for each option you choose.
These directions are subject to change as new devices are released, but should be similar.